1. Field of the Invention
The present invention is directed to a seal element consisting of elastic material suitable for use in an implantable medical apparatus.
2. Description of the Prior Art
A heart pacemaker is described in U.S. Pat. 4,141,752 as exemplifying an implantable medical apparatus of the type wherein a seal element must be provided. For fastening an electrode lead to the terminal portion of heart pacemaker, the terminal portions provided with a hole or bore in which a screw is received which fixes the proximal end of the lead in the terminal portion. The screw is provided with an elastic seal element which seals the opening of the hole. A screwdriver which penetrates the seal element in order to reach the screw is used for turning the screw. A disadvantage of this conventional seal element is that the point of penetration does not always close upon removal of the screwdriver, so that the integrity of the seal is no longer guaranteed, and body fluid may penetrate into the terminal portion. This may result in leakage currents so that the energy of the stimulation pulses, which are supplied to the patient via the distal end of the lead, is partially lost, and may no longer be sufficient for stimulation of the heart under certain circumstances. Additionally due to the leakage currents, tissue in the region of the terminal portion may be undesirably stimulated, resulting in muscle spasms which are uncomfortable for the patient. Moreover, due to the leakage currents, the sensing function of the pacemaker electrode is degraded. If the leak is opened and closed in sequence, for example due to physical movements, the pacemaker may incorrectly interpret this as a QRS complex or a P-wave, which may result in inhibition of the generation of stimulation pulses under certain circumstances, if the pacemaker is a demand pacer. Moreover, if the seal is not tight, blood may penetrate into the region surrounding the screw, and coagulate, so that it is difficult to loosen the screw if the pacemaker must be subsequently replaced.